In characterizing the cell-cycle regulatory role of the cyclin-dependent kinase inhibitor, p27kip1, it was discovered that p27 knockout mice exhibit pronounced multi-organomegaly with no increased spontaneous carcinogenesis in the liver. This suggested that p27 manipulation could improve the success of hepatocyte transplantation. It was then shown that donor hepatocytes from p27 knockout mice more actively proliferate and rescue liver failure in the FAH mouse model. These results were limited by the fact that p27 was suppressed throughout the donor animal from early embryogenesis and the risk of carcinogenesis due to longterm p27 absence was not determined. In this proposal we develop two new techniques for targeting p27 in the mouse liver. Mx 1-Cre transgenic mice will allow us to knockout p27 in the liver alone at controlled time points of hepatogenesis. Ectopic expression of the F-box protein Skp2 from Alumin promoter will allow us to lower p27 protein abundance in the hepatocytes. We also propose to inactivate p27 through lentivirus mediated delivery of Skp2 and p27 specific shRNA, which can be directly applicable to human hepatocytes in the future. With these new approaches of p27 inactivation, we will determine the role of p27 in hepatocyte transplantation and hepatocellular carcinogenesis in more clinically relevant settings. The results will give us a clear assessment of the potential benefits of p27 suppression in the treatment of human liver failure.